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diff --git a/lib/rubygems/requirement.rb b/lib/rubygems/requirement.rb
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+++ b/lib/rubygems/requirement.rb
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+# frozen_string_literal: true
+
+require_relative "version"
+
+##
+# A Requirement is a set of one or more version restrictions. It supports a
+# few (<tt>=, !=, >, <, >=, <=, ~></tt>) different restriction operators.
+#
+# See Gem::Version for a description on how versions and requirements work
+# together in RubyGems.
+
+class Gem::Requirement
+  OPS = { # :nodoc:
+    "=" => lambda {|v, r| v == r },
+    "!=" => lambda {|v, r| v != r },
+    ">" => lambda {|v, r| v > r },
+    "<" => lambda {|v, r| v < r },
+    ">=" => lambda {|v, r| v >= r },
+    "<=" => lambda {|v, r| v <= r },
+    "~>" => lambda {|v, r| v >= r && v.release < r.bump },
+  }.freeze
+
+  SOURCE_SET_REQUIREMENT = Struct.new(:for_lockfile).new "!" # :nodoc:
+
+  quoted = Regexp.union(OPS.keys)
+  PATTERN_RAW = "\\s*(#{quoted})?\\s*(#{Gem::Version::VERSION_PATTERN})\\s*".freeze # :nodoc:
+
+  ##
+  # A regular expression that matches a requirement
+
+  PATTERN = /\A#{PATTERN_RAW}\z/
+
+  ##
+  # The default requirement matches any non-prerelease version
+
+  DefaultRequirement = [">=", Gem::Version.new(0)].freeze
+
+  ##
+  # The default requirement matches any version
+
+  DefaultPrereleaseRequirement = [">=", Gem::Version.new("0.a")].freeze
+
+  ##
+  # Raised when a bad requirement is encountered
+
+  class BadRequirementError < ArgumentError; end
+
+  ##
+  # Factory method to create a Gem::Requirement object.  Input may be
+  # a Version, a String, or nil.  Intended to simplify client code.
+  #
+  # If the input is "weird", the default version requirement is
+  # returned.
+
+  def self.create(*inputs)
+    return new inputs if inputs.length > 1
+
+    input = inputs.shift
+
+    case input
+    when Gem::Requirement then
+      input
+    when Gem::Version, Array then
+      new input
+    when "!" then
+      source_set
+    else
+      if input.respond_to? :to_str
+        new [input.to_str]
+      else
+        default
+      end
+    end
+  end
+
+  def self.default
+    new ">= 0"
+  end
+
+  def self.default_prerelease
+    new ">= 0.a"
+  end
+
+  ###
+  # A source set requirement, used for Gemfiles and lockfiles
+
+  def self.source_set # :nodoc:
+    SOURCE_SET_REQUIREMENT
+  end
+
+  ##
+  # Parse +obj+, returning an <tt>[op, version]</tt> pair. +obj+ can
+  # be a String or a Gem::Version.
+  #
+  # If +obj+ is a String, it can be either a full requirement
+  # specification, like <tt>">= 1.2"</tt>, or a simple version number,
+  # like <tt>"1.2"</tt>.
+  #
+  #     parse("> 1.0")                 # => [">", Gem::Version.new("1.0")]
+  #     parse("1.0")                   # => ["=", Gem::Version.new("1.0")]
+  #     parse(Gem::Version.new("1.0")) # => ["=,  Gem::Version.new("1.0")]
+
+  def self.parse(obj)
+    return ["=", obj] if Gem::Version === obj
+
+    unless PATTERN =~ obj.to_s
+      raise BadRequirementError, "Illformed requirement [#{obj.inspect}]"
+    end
+    op = -($1 || "=")
+    version = -$2
+
+    if op == ">=" && version == "0"
+      DefaultRequirement
+    elsif op == ">=" && version == "0.a"
+      DefaultPrereleaseRequirement
+    else
+      [op, Gem::Version.new(version)]
+    end
+  end
+
+  ##
+  # An array of requirement pairs. The first element of the pair is
+  # the op, and the second is the Gem::Version.
+
+  attr_reader :requirements # :nodoc:
+
+  ##
+  # Constructs a requirement from +requirements+. Requirements can be
+  # Strings, Gem::Versions, or Arrays of those. +nil+ and duplicate
+  # requirements are ignored. An empty set of +requirements+ is the
+  # same as <tt>">= 0"</tt>.
+
+  def initialize(*requirements)
+    requirements = requirements.flatten
+    requirements.compact!
+    requirements.uniq!
+
+    if requirements.empty?
+      @requirements = [DefaultRequirement]
+    else
+      @requirements = requirements.map! {|r| self.class.parse r }
+    end
+  end
+
+  ##
+  # Concatenates the +new+ requirements onto this requirement.
+
+  def concat(new)
+    new = new.flatten
+    new.compact!
+    new.uniq!
+    new = new.map {|r| self.class.parse r }
+
+    @requirements.concat new
+  end
+
+  ##
+  # Formats this requirement for use in a Gem::RequestSet::Lockfile.
+
+  def for_lockfile # :nodoc:
+    return if @requirements == [DefaultRequirement]
+
+    list = requirements.sort_by do |_, version|
+      version
+    end.map do |op, version|
+      "#{op} #{version}"
+    end.uniq
+
+    " (#{list.join ", "})"
+  end
+
+  ##
+  # true if this gem has no requirements.
+
+  def none?
+    if @requirements.size == 1
+      @requirements[0] == DefaultRequirement
+    else
+      false
+    end
+  end
+
+  ##
+  # true if the requirement is for only an exact version
+
+  def exact?
+    return false unless @requirements.size == 1
+    @requirements[0][0] == "="
+  end
+
+  def as_list # :nodoc:
+    requirements.map {|op, version| "#{op} #{version}" }
+  end
+
+  def hash # :nodoc:
+    requirements.map {|r| r.first == "~>" ? [r[0], r[1].to_s] : r }.sort.hash
+  end
+
+  def marshal_dump # :nodoc:
+    [@requirements]
+  end
+
+  def marshal_load(array) # :nodoc:
+    @requirements = array[0]
+
+    raise TypeError, "wrong @requirements" unless Array === @requirements &&
+                                                  @requirements.all? {|r| r.size == 2 && (r.first.is_a?(String) || r[0] = "=") && r.last.is_a?(Gem::Version) }
+  end
+
+  def yaml_initialize(tag, vals) # :nodoc:
+    vals.each do |ivar, val|
+      instance_variable_set "@#{ivar}", val
+    end
+  end
+
+  def init_with(coder) # :nodoc:
+    yaml_initialize coder.tag, coder.map
+  end
+
+  def encode_with(coder) # :nodoc:
+    coder.add "requirements", @requirements
+  end
+
+  ##
+  # A requirement is a prerelease if any of the versions inside of it
+  # are prereleases
+
+  def prerelease?
+    requirements.any? {|r| r.last.prerelease? }
+  end
+
+  def pretty_print(q) # :nodoc:
+    q.group 1, "Gem::Requirement.new(", ")" do
+      q.pp as_list
+    end
+  end
+
+  ##
+  # True if +version+ satisfies this Requirement.
+
+  def satisfied_by?(version)
+    raise ArgumentError, "Need a Gem::Version: #{version.inspect}" unless
+      Gem::Version === version
+    requirements.all? {|op, rv| OPS.fetch(op).call version, rv }
+  end
+
+  alias_method :===, :satisfied_by?
+  alias_method :=~, :satisfied_by?
+
+  ##
+  # True if the requirement will not always match the latest version.
+
+  def specific?
+    return true if @requirements.length > 1 # GIGO, > 1, > 2 is silly
+
+    !%w[> >=].include? @requirements.first.first # grab the operator
+  end
+
+  def to_s # :nodoc:
+    as_list.join ", "
+  end
+
+  def ==(other) # :nodoc:
+    return unless Gem::Requirement === other
+
+    # An == check is always necessary
+    return false unless _sorted_requirements == other._sorted_requirements
+
+    # An == check is sufficient unless any requirements use ~>
+    return true unless _tilde_requirements.any?
+
+    # If any requirements use ~> we use the stricter `#eql?` that also checks
+    # that version precision is the same
+    _tilde_requirements.eql?(other._tilde_requirements)
+  end
+
+  protected
+
+  def _sorted_requirements
+    @_sorted_requirements ||= requirements.sort_by(&:to_s)
+  end
+
+  def _tilde_requirements
+    @_tilde_requirements ||= _sorted_requirements.select {|r| r.first == "~>" }
+  end
+
+  def initialize_copy(other) # :nodoc:
+    @requirements = other.requirements.dup
+    super
+  end
+end
+
+class Gem::Version
+  # This is needed for compatibility with older yaml
+  # gemspecs.
+
+  Requirement = Gem::Requirement # :nodoc:
+end